This invention relates to improvements in an ultrasonic measurement method for subjecting an object to an ultrasonic transmission and receiving reflected ultrasonic waves from the interior of the object to measure the acoustic characteristics of the object. More particularly, the invention relates to an ultrasonic measurement method and apparatus for providing information relating to attenuation that accompanies propagation of ultrasonic waves in the interior of an object.
Ultrasonic measurement techniques find application widely in such fields as material testing, SONAR and medical diagnosis. In particular, ultrasound scanner systems for medical purposes have recently been developed.
The principle of operation of an ultrasound scanner apparatus resides in use of a pulse-echo method and utilizes a phenomenon wherein an ultrasonic pulse transmitted into a living body, which is the object undergoing measurement, is reflected at a boundary where there is a difference in acoustic impedence.
The reflected wave (echo) is received and processed to display a tomograph of the living body by a so-called B-mode method. The echo contains a variety of information such as the ultrasonic attenuation, acoustic impedence and propagation velocity of sound. With an apparatus of this kind, however, these various items of information cannot be separated from one another distinctly, so that the general practice is merely to display the amplitude of the echo.
More specifically, the propagation velocity of sound is assumed to be constant and, with regard to attenuation ascribable to ultrasonic propagation, the value of echo amplitude arbitrarily corrected by a so-called STC (sensitivity time control) circuit is luminance-modulated, with the modulated amplitude being displayed on a cathode-ray tube as a tomograph. Accordingly, the tomograph obtained is a two-dimensional distribution at the acoustic impedence interface rendered qualitatively into a picture, so that the morphological information relating to the position and shape of the bioligical tissue inevitably forms the core of the information utilized. In other words, the state of the art is such that such biological tissue characteristics as degree of attenuation and propagation velocity of sound are not measured.
Several attempts at attaining attenuation information relating to biological tissue have been reported. As will be described below in further detail, an echo signal contains two types of information, namely attenuation due to propagation through biological tissue, and intensity of reflection at an interface or boundary where there is a difference in acoustic impedence. Both of these quantities are unknown. Therefore, isolating the affects of these two quantities is extremely difficult at the present time.
If the reflected intensity is assumed to be independent of the frequency of the ultrasonic waves and ultrasonic waves having a plurality of different frequencies are transmitted and received with regard to the same portion of the object under measurement followed by measuring the sound pressure ratio of each frequency component of the echo, then it will be possible to eliminate the influence of the reflected intensity and derive an attenuation coefficient ascribable to propagation without the affects of the reflected intensity. The foregoing assumption holds in the case of an acoustic boundary having a sufficiently wide range in comparison with the wavelength of the ultrasonic waves, e.g. in the case of a planar reflector. In actuality, however, a scatterer having a size approximately equivalent to or less than the wavelengths used can reside at the biological tissue. It is therefore difficult to consider that the foregoing assumption will always hold for an entire bioligical tissue.
In view of the foregoing circumstances, the applicant has proposed, in Japanese Patent Application Laid-Open No.56-147082, a method and apparatus for measuring attenuation information ascribable to ultrasonic propagation in a living body in which the influence of reflected intensity at an acoustic boundary is reduced. However, the ultrasonic attenuation information obtained according to the invention of this earlier application is not an attenuation coefficient but an equation of the first degree involving a mean value of ultrasonic attenuation coefficients over a prescribed interval.